Menghrajani Kishan S, Barnes William L
Department of Physics and Astronomy, University of Exeter, Exeter, EX4 4QL, United Kingdom.
ACS Photonics. 2020 Sep 16;7(9):2448-2459. doi: 10.1021/acsphotonics.0c00552. Epub 2020 Aug 6.
Strong coupling of molecules placed in an optical microcavity may lead to the formation of hybrid states called polaritons; states that inherit characteristics of both the optical cavity modes and the molecular resonance. Developing a better understanding of the matter characteristics of these hybrid states has been the focus of much recent attention. Here, as we will show, a better understanding of the role of the optical modes supported by typical cavity structures is also required. Typical microcavities used in molecular strong coupling experiments support more than one mode at the frequency of the material resonance. While the effect of strong coupling to multiple photonic modes has been considered before, here we extend this topic by looking at strong coupling between one vibrational mode and multiple photonic modes. Many experiments involving strong coupling make use of metal-clad microcavities, ones with metallic mirrors. Metal-clad microcavities are well-known to support coupled plasmon modes in addition to the standard microcavity mode. However, the coupled plasmon modes associated with a metal-clad optical microcavity lie beyond the light-line and are thus not probed in typical experiments on strong coupling. Here we investigate, through experiment and numerical modeling, the interaction between molecules within a cavity and the modes both inside and outside the light-line. Making use of grating coupling and a metal-clad microcavity, we provide an experimental demonstration that such modes undergo strong coupling. We further show that a common variant of the metal-clad microcavity, one in which the metal mirrors are replaced by distributed Bragg reflector also show strong coupling to modes that exist in these structures beyond the light-line. Our results highlight the need to consider the effect of beyond the light-line modes on the strong coupling of molecular resonances in microcavities and may be of relevance in designing strong coupling resonators for chemistry and materials science investigations.
置于光学微腔中的分子的强耦合可能导致形成称为极化激元的混合态；这些态继承了光学腔模和分子共振的特征。更好地理解这些混合态的物质特性一直是近期备受关注的焦点。正如我们将展示的，这里还需要更好地理解典型腔结构所支持的光学模式的作用。分子强耦合实验中使用的典型微腔在材料共振频率下支持不止一种模式。虽然之前已经考虑过与多个光子模式的强耦合效应，但在这里我们通过研究一个振动模式与多个光子模式之间的强耦合来扩展这个主题。许多涉及强耦合的实验使用金属包覆微腔，即带有金属镜的微腔。众所周知，金属包覆微腔除了支持标准的微腔模式外，还支持耦合等离子体模式。然而，与金属包覆光学微腔相关的耦合等离子体模式位于光线之外，因此在典型的强耦合实验中无法探测到。在这里，我们通过实验和数值模拟研究腔内分子与光线内外模式之间的相互作用。利用光栅耦合和金属包覆微腔，我们提供了一个实验证明，表明这些模式会发生强耦合。我们进一步表明，金属包覆微腔的一种常见变体，即金属镜被分布式布拉格反射器取代的微腔，也显示出与这些结构中存在于光线之外的模式的强耦合。我们的结果强调了需要考虑光线之外的模式对微腔中分子共振强耦合的影响，这可能与设计用于化学和材料科学研究的强耦合谐振器相关。
